Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg

   1 What:           /sys/firmware/qemu_fw_cfg/
   2 Date:           August 2015
   3 Contact:        Gabriel Somlo <somlo@cmu.edu>
   4 Description:
   5                 Several different architectures supported by QEMU (x86, arm,
   6                 sun4*, ppc/mac) are provisioned with a firmware configuration
   7                 (fw_cfg) device, originally intended as a way for the host to
   8                 provide configuration data to the guest firmware. Starting
   9                 with QEMU v2.4, arbitrary fw_cfg file entries may be specified
  10                 by the user on the command line, which makes fw_cfg additionally
  11                 useful as an out-of-band, asynchronous mechanism for providing
  12                 configuration data to the guest userspace.
  13
  14                 The authoritative guest-side hardware interface documentation
  15                 to the fw_cfg device can be found in "docs/specs/fw_cfg.txt"
  16                 in the QEMU source tree.
  17
  18                 === SysFS fw_cfg Interface ===
  19
  20                 The fw_cfg sysfs interface described in this document is only
  21                 intended to display discoverable blobs (i.e., those registered
  22                 with the file directory), as there is no way to determine the
  23                 presence or size of "legacy" blobs (with selector keys between
  24                 0x0002 and 0x0018) programmatically.
  25
  26                 All fw_cfg information is shown under:
  27
  28                         /sys/firmware/qemu_fw_cfg/
  29
  30                 The only legacy blob displayed is the fw_cfg device revision:
  31
  32                         /sys/firmware/qemu_fw_cfg/rev
  33
  34                 --- Discoverable fw_cfg blobs by selector key ---
  35
  36                 All discoverable blobs listed in the fw_cfg file directory are
  37                 displayed as entries named after their unique selector key
  38                 value, e.g.:
  39
  40                         /sys/firmware/qemu_fw_cfg/by_key/32
  41                         /sys/firmware/qemu_fw_cfg/by_key/33
  42                         /sys/firmware/qemu_fw_cfg/by_key/34
  43                         ...
  44
  45                 Each such fw_cfg sysfs entry has the following values exported
  46                 as attributes:
  47
  48                 name    : The 56-byte nul-terminated ASCII string used as the
  49                           blob's 'file name' in the fw_cfg directory.
  50                 size    : The length of the blob, as given in the fw_cfg
  51                           directory.
  52                 key     : The value of the blob's selector key as given in the
  53                           fw_cfg directory. This value is the same as used in
  54                           the parent directory name.
  55                 raw     : The raw bytes of the blob, obtained by selecting the
  56                           entry via the control register, and reading a number
  57                           of bytes equal to the blob size from the data
  58                           register.
  59
  60                 --- Listing fw_cfg blobs by file name ---
  61
  62                 While the fw_cfg device does not impose any specific naming
  63                 convention on the blobs registered in the file directory,
  64                 QEMU developers have traditionally used path name semantics
  65                 to give each blob a descriptive name. For example:
  66
  67                         "bootorder"
  68                         "genroms/kvmvapic.bin"
  69                         "etc/e820"
  70                         "etc/boot-fail-wait"
  71                         "etc/system-states"
  72                         "etc/table-loader"
  73                         "etc/acpi/rsdp"
  74                         "etc/acpi/tables"
  75                         "etc/smbios/smbios-tables"
  76                         "etc/smbios/smbios-anchor"
  77                         ...
  78
  79                 In addition to the listing by unique selector key described
  80                 above, the fw_cfg sysfs driver also attempts to build a tree
  81                 of directories matching the path name components of fw_cfg
  82                 blob names, ending in symlinks to the by_key entry for each
  83                 "basename", as illustrated below (assume current directory is
  84                 /sys/firmware):
  85
  86                     qemu_fw_cfg/by_name/bootorder -> ../by_key/38
  87                     qemu_fw_cfg/by_name/etc/e820 -> ../../by_key/35
  88                     qemu_fw_cfg/by_name/etc/acpi/rsdp -> ../../../by_key/41
  89                     ...
  90
  91                 Construction of the directory tree and symlinks is done on a
  92                 "best-effort" basis, as there is no guarantee that components
  93                 of fw_cfg blob names are always "well behaved". I.e., there is
  94                 the possibility that a symlink (basename) will conflict with
  95                 a dirname component of another fw_cfg blob, in which case the
  96                 creation of the offending /sys/firmware/qemu_fw_cfg/by_name
  97                 entry will be skipped.
  98
  99                 The authoritative list of entries will continue to be found
 100                 under the /sys/firmware/qemu_fw_cfg/by_key directory.